The present invention relates to minimum-shift keying (MSK) modulators, and more particularly to a technique for producing MSK modulation at the output of a voltage-controlled oscillator ("VCO").
Previous circuits for the production of MSK have relied on synthesizing the in-phase and quadrature signals independently, or on Surface Acoustic Wave (SAW) filters that typically require subsequent up-conversion (or down conversion) to the radio frequency (RF) carrier of interest. An exemplary circuit is described in F. Amoroso and J. A. Kivett, "Simplified MSK signaling technique," IEEE Transactions on Communications, Vol. COM-25, No. 4, pp. 433-441, April 1977.
It is an object of the invention to produce MSK modulation using a VCO output directly at the required RF carrier.
A further object of the invention is to provide a modulator circuit employing a VCO capable of producing modified MSK modulation at the VCO output as the result of low pass filtering prior to the VCO to reduce power spectral sidelobes in the VCO signal. Exemplary low pass filtering and spectral sidelobe reduction are shown in F. Amoroso, "Loss of SNR in low pass filtered FM/MSK," IEEE MILCOM '86, paper 40.2, Monterey, CA, Oct. 5-9, 1986.